1. Field of the Invention
This invention relates to metallic and intermetallic composite materials having embedded therein fiber optical cables for sensing and monitoring environmental and structural disturbances or effects upon the composite itself, and this invention relates to methods for manufacturing such composites. The composite materials are of the type used to construct aircraft structures, particularly the skin or surface of aircraft, including airplanes, missiles, or trans-atmospheric vehicles.
2. Description of the Prior Art
Ultrastructure research into advanced composite material structures for trans-atmospheric vehicles, missiles, and other advanced high-speed aircraft, particularly for their outer skin construction has been progressing at a rapid pace. Polymer-matrix reinforced composite materials, carbon fibers spun from organic polymer precursors, solid blocks of ceramic material, super sol-gels and molecular beam epitaxy are among the technologies for such ultrastructure research. These investigations involve control and positioning of load bearing fibers for reinforcement purposes and advanced molecular structuring for the materials employed in aircraft construction.
Although materials technology has expanded from glass fiber-polymer resin composites to certain metals, ceramics and polymers in a variety of forms for both reinforcing load-bearing fibers and matrix applications, there is a continually increasing need for improving the performance of intermetallics and matching their metallic properties to those of adjacent composite materials and structures. At present, graphite epoxy is the most common example of preformed composite material used in testing advanced material applications for aircraft structures. However, the mechanical complexity of other advanced composite materials, particularly titanium composites, is matched by the variety of defects that may occur. Engineering experience with these materials has not yet progressed to the extent that there is confidence in predicting safe service lives, given even minor defects, particularly the problems and questions associated with their compatibility to adjacent substrates. See, American Institute of Aeronautics and Astronautics Second Aerospace Maintenance Conference publication, May 21-23, 1986/San Antonio, Texas (AIAA-86-1139). Additionally, information is available in Aviation Week & Space Technology/Jul. 7, 1986, page 105, where the article DARPA, USAF Develop Advance Materials Processing Technology by Mr. Jay C. Lowndes is printed.
One current focus of ultrastructure composite materials research is that of embedded optical fiber sensors fabricated as an integral part of a composite structure. Such fibers could be used to measure uniaxial strain along the fiber, to detect high frequency dynamic strains in the composite material, to transmit acoustic emissions, to detect and characterize damage as it happens in advanced composite materials, and to measure the affects of electric fields, magnetic fields, pressure, and/or temperature on the composite material. In fact, it is desired to develop composite materials having integral optical fiber sensors that would enable the production of a full-up electronic device embedded in an aircraft skin, including, for example, antennas, or other elements where optical light waves travelling through the fiber may be modulated and, in the absence of extraneous optical distortions, they can be interpreted into electronic signals on the basis of various phase changes. Therefore, the environmental and structural disturbances on the composite material forming the skin of an aircraft, can be detected and monitored.
Although limited success has been achieved in demonstrations of optical fibers embedded in graphite epoxy test specimens, the trend to test fiber reinforced composite structures in advanced aircraft or space vehicles has overtaken the data basis, analytical methods, and engineering procedures to the extent that little assurance and/or success has been achieved with metal alloy composites currently in service, particularly, in the case of non-homogeneous composite materials. One such currently in-service composite for which it would be highly desirable to embed optical fibers is titanium matrix composite material.
It is known that titanium molecules attack the silica present in optical fibers so as to alter its refractive index. Although this effect has been used to the advantage of manufacturing certain types of optical fiber material as, for example, in U.S. Pat. No. 2,272,342, where optical fibers were made lossy by virtue of the fact that Ti.sup.4+ reduced to Ti.sup.3+ absorbs light in the red and infrared spectral regions, the diffusion of titanium molecules from titanium matrix composites adjacent to an optical fiber, distorts or prohibits the transmittance of a light's signal and/or its refractive index so as to breakdown the optical fiber and render it ineffective as a means of sensing the disturbances on the skins of aircraft.
A titanium matrix composite ultrastructure material having a distortion-free optical fiber integrally embedded therein, effective in sensing the environmental disturbances on the composite material, and a method for manufacturing the same would fulfill a long felt need in the industry and would represent a tremendous advancement in the art.